Postsynaptic Receptor Clusters Research Articles

Strychnine-sensitive stabilization of postsynaptic glycine receptor clusters.

The cellular and molecular mechanisms underlying the postsynaptic aggregation of ionotropic receptors in the central nervous system are not understood. The glycine receptor (GlyR) and its cytoplasmic domain-associated protein, gephyrin, are clustered at the postsynaptic membrane and constitute a good model for addressing these questions. The glycine receptor is inhibited by strychnine. The effects of chronic strychnine treatment on the expression and cellular distribution of gephyrin and glycine receptor were therefore tested using primary cultures of spinal cord neurons. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed that the glycine receptor alpha1, alpha2, beta subunits and gephyrin mRNAs were expressed at comparable levels in strychnine-treated and untreated cultures. The number of immunoreactive cells and the subcellular distribution of gephyrin and GlyR subunits was determined with standard and confocal immunofluorescence. The proportion of gephyrin and glycine receptor-immunoreactive (IR) cells was unaffected by strychnine treatment. Confocal microscopy revealed that the glycine receptor was mainly localized intracellularly near the nucleus. This cytoplasmic glycine receptor was not associated with the Golgi apparatus nor with the rough endoplasmic reticulum and therefore is not likely to correspond to neosynthesized proteins. The number of GlyR clusters on the somato-dendritic membrane was dramatically reduced on neurons displaying intracellular staining. In contrast, the subcellular distribution and the number of gephyrin clusters was not modified by the treatment. The fact that gephyrin postsynaptic localization was not modified by strychnine suggests that the aggregation of glycine receptor and gephyrin is governed by different mechanisms. The distribution of other cell surface molecules such as NCAM or GABAA receptor beta2/3 subunits was not modified by strychnine treatment. Chronic exposure of the cultures to tetrodotoxin did not affect gephyrin or glycine receptor cluster formation. Taken together, these results indicate that functional glycine receptor, but not electrical synaptic activity, is required for the formation of glycine receptor clusters.

Clustering of rapsyn is regulated by rho in fibroblast

Glutamate plays a predominant role in regulating the activity of orexin neurons that coordinate motivated behaviors, sleep-wake cycle and autonomic functions. To gain more insight into the properties of excitatory transmission to orexin neurons, whole cell patch clamp recordings were made in rat brain slices and quantal analysis of pharmacologically isolated miniature excitatory postsynaptic currents (mEPSCs) was performed. In more than half the orexin neurons examined, mEPSCs showed heterogeneous time course: some mEPSCs had fast rise and decay (fast mEPSC), while some had longer kinetics, smaller amplitude but larger integrated area (slow mEPSC). Other orexin neurons showed low frequency mEPSCs with uniform, fast kinetics. In the former, distribution histogram of 10–90% rise time displayed two peaks, indicating that fast and slow mEPSCs are distinct subgroups. Occasionally fast and slow EPSCs would summate, suggesting that they arise from different pairs of active zones and postsynaptic receptor clusters. A large majority of mEPSCs were mediated by AMPA receptors that are sensitive to GYKI 52466 and DNQX. To determine whether synapses that give rise to fast and slow mEPSCs are differentially modulated, the D1- and D2-like agonists were tested on various parameters of mEPSCs. The agonists altered the frequency as previously reported, but had no effect on the rise, decay or area of mEPSC, suggesting that dopamine affects fast and slow mEPSCs equally. Given the potential physiological impact of EPSC time course on synaptic integration, our study raises an interesting possibility that distinct subset of excitatory synaptic inputs are processed differently by orexin neurons.

The Development of Excitatory Synapses in Cultured Spinal Neurons

Immunohistochemical studies of synapses in the CNS have demonstrated that glutamate receptors (GluRs) are concentrated at postsynaptic sites in vivo and in vitro (Baude et al., 1995). The mechanisms leading to receptor clustering at excitatory synapses are far less understood than those governing acetylcholine receptor accumulation at the neuromuscular junction () or glycine receptor aggregation at central inhibitory synapses (). Using cultured rat spinal cord neurons, we demonstrate that clustering of the AMPA receptor subunit GluR1 is among the earliest events in excitatory synapse formation in vitro, coincident with the onset of miniature EPSCs and in many cases preceding presynaptic vesicle accumulation. Postsynaptic receptor clustering is induced in a highly specific and reiterative pattern, independent of receptor activation, by contact with a subset of axons capable of inducing receptor clusters. The subunit composition of AMPA receptor clusters varied significantly between neurons but was invariant within a given neuron. The presence of either GluR2 or GluR3 was common to all receptor clusters. Neither high-affinity glutamate transporters nor NMDA receptors appeared to be concentrated with AMPA receptor subunits at these excitatory synapses.

Synaptogenesis in the rat retina: subcellular localization of glycine receptors, GABAA receptors, and the anchoring protein gephyrin

The mechanisms by which neurotransmitter receptors are clustered at postsynaptic sites of neurons are largely unknown. The 93-kDa peripheral membrane protein gephyrin has been shown to be essential for the formation of postsynaptic glycine receptor clusters, and there is now evidence that gephyrin can also be found at gamma-aminobutyric acid (GABA)ergic synapses. In this study, we have analyzed the synaptic localization of glycine receptors, GABA(A) receptors, and the anchoring protein gephyrin in the inner plexiform layer of the developing rat retina, by using immunofluorescence with subunit specific antibodies. At early postnatal stages, the antibodies produced a diffuse staining, suggesting that early retinal neurons can express glycine and GABA(A) receptors. A clustered distribution of the subunits in "hot spots" was also observed. The number of "hot spots" increased during development and reached adult levels in about 2 weeks. Electron microscopy showed that synapses of the conventional type are present in the inner plexiform layer of the postnatal retina and that the hot spots correspond to an aggregation of receptors at postsynaptic sites. Gephyrin was also localized to "hot spots," and double immunofluorescence revealed a colocalization of gephyrin with the alpha2 subunit of the GABA(A) receptor. These results indicate that clustering of receptor subunits occurs in parallel with the formation of morphologically identifiable synaptic specializations and suggest that gephyrin may be involved in clustering of GABA(A) receptors at postsynaptic sites.

Cell-type specific organization of glycine receptor clusters in the mammalian spinal cord

Glycinergic synapses play a major role in shaping the activity of spinal cord neurons. The spatial organization of postsynaptic receptors is likely to determine many functional parameters at these synapses and is probably related to the integrative capabilities of different neurons. In the present study, we have investigated the organization of gephyrin expression along the dendritic membranes of alpha- and gamma-motoneurons, Ia inhibitory interneurons, and Renshaw cells. Gephyrin is a protein responsible for the postsynaptic clustering of glycine receptors, and the features of gephyrin and glycine receptor alpha(1)-subunit immunofluorescent clusters displayed similar characteristics on ventral horn spinal neurons. However, the density of clusters and their topographical organization and architecture varied widely in different neurons and in different dendritic regions. For motoneurons and Ia inhibitory interneurons, cluster size and complexity increased with distance from the soma, perhaps as a mechanism to enhance the influence of distal synapses. Renshaw cells were special in that they displayed an abundant complement of large and morphologically complex clusters concentrated in their somas and proximal dendrites. Serial electron microscopy confirmed that the various immunoreactivity patterns observed with immunofluorescence accurately parallel the variable organization of pre- and postsynaptic active zones of glycinergic synapses. Finally, synaptic boutons from single-labeled axons of glycinergic neurons (Ia inhibitory interneurons) were also associated with postsynaptic receptor clusters of variable shapes and configurations. Our results indicate that mechanisms regulating receptor clustering do so primarily in the context of the postsynaptic neuron identity and localization in the dendritic arbor.

Analytical description of the activation of multi-state receptors by continuous neurotransmitter signals at brain synapses

Chemical synaptic transmission is a fundamental component of interneuronal communications in the central nervous system (CNS). Discharge of a presynaptic vesicle containing a few thousand molecules (a quantum) of neurotransmitter into the synaptic cleft generates a transmitter concentration signal that drives postsynaptic ion-channel receptors. These receptors exhibit multiple states, with state transition kinetics dependent on neurotransmitter concentration. Here, a novel and simple analytical approach for describing gating of multi-state receptors by signals with complex continuous time courses is used to describe the generation of glutamate-mediated quantal postsynaptic responses at brain synapses. The neurotransmitter signal, experienced by multi-state N-methyl-D-aspartate (NMDA)- and L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors at specific points in a synaptic cleft, is approximated by a series of step functions of different intensity and duration and used to drive a Markovian, multi-state kinetic scheme that describes receptor gating. Occupancy vectors at any point in time can be computed interatively from the occupancy vectors at the times of steps in transmitter concentration. Multi-state kinetic schemes for both the low-affinity AMPA subtype of glutamate receptor and for the high-affinity NMDA subtype are considered, and expected NMDA and AMPA components of synaptic currents are calculated. The amplitude of quantal responses mediated by postsynaptic receptor clusters having specific spatial distributions relative to foci of quantal neurotransmitter release is then calculated and related to the displacement between the center of the postsynaptic receptor cluster and the focus of synaptic vesicle discharge. Using this approach we show that the spatial relation between the focus of release and the center of the postsynaptic receptor cluster affects synaptic efficacy. We also show how variation in this relation contributes to variation in synaptic current amplitudes.

A mathematical description of miniature postsynaptic current generation at central nervous system synapses

Variation in the amplitude of miniature postsynaptic currents (mPSCs) generated by individual quanta of neurotransmitter is a major contributor to the variance of evoked synaptic responses. Here we explore the possible origins of this variability by developing a mathematical description of mPSC generation and consider the contribution of "off-center" release to this variability. By "off-center" release we mean variation in the distance between the position where a presynaptic vesicle discharges its content of neurotransmitter into the synaptic cleft and the center of a cluster of postsynaptic receptors (PRCs) that responds to those transmitter molecules by generating an mPSC. We show that when the time course of quantal discharge through a fusion pore (noninstantaneous release) is considered, elementary analytical descriptions of the subsequent diffusion of transmitter within the synaptic cleft (with or without uptake) predict the development of significant gradients of transmitter concentration during the rising phase of mPSCs. This description of diffusion is combined with a description of the pharmacodynamics of receptors in the PRC and of the time dependence of the gradient of transmitter concentration over the area of the PRC to reconstruct the time course and amplitude of an mPSC for a synapse of a given geometry. Within the constraints of known dimensions of presynaptic active zones and postsynaptic receptor clusters at CNS synapses, our analysis suggests that "off-center" release, produced by allowing release to occur anywhere within an anatomically defined presynaptic active zone, can be an important contributor to mPSC variability. Indeed, modulation of the influence of "off-center" release may be a novel way of controlling synaptic efficacy. We also show how noninstantaneous release can serve to focus the action of neurotransmitter within a given synapse and thereby reduce cross-talk between synapses.

The inhibitory glycine receptor: architecture, synaptic localization and molecular pathology of a postsynaptic ion-channel complex

Significant progress has been made towards the identification of functional domains of the inhibitory glycine receptor. Several residues crucial for ligand binding, ion-channel properties and stoichiometric subunit assembly have been identified. A major recent advance has been the finding that the biogenesis of postsynaptic glycine receptor clusters requires the tubulin-binding protein, gephyrin. Another area of exciting research has focused on mutations of glycine receptor α and β subunit genes, which have been found to be causal for different hereditary motor disorders.

The postsynaptic localization of the glycine receptor-associated protein gephyrin is regulated by the cytoskeleton

The mechanisms underlying the postsynaptic localization of neurotransmitter receptors are poorly understood. Recently, the peripheral membrane protein gephyrin has been shown to be essential for the formation of inhibitory glycine receptor clusters in cultured rat spinal cord neurons. In vitro gephyrin binds with high affinity to polymerized tubulin. Here, the interaction of gephyrin with different components of the cytoskeleton was investigated in primary cultures of rat spinal neurons. After treatment with alkaloids affecting the cytoskeleton, the morphology of post-synaptic gephyrin clusters was analyzed by confocal immunofluorescence microscopy. Depolymerization of microtubules by demecolcine reduced both the percentage of cells with postsynaptic gephyrin clusters and the number of clusters/cell. The size of the remaining gephyrin clusters was increased whereas their gephyrin density was significantly lower than under control conditions. Depolymerization of microfilaments by cytochalasin D in contrast generated smaller clusters of increased gephyrin density. Demecolcine also dispersed postsynaptic glycine receptor clusters as revealed by immunostaining with a specific monoclonal antibody. These findings support the view that in vivo gephyrin anchors receptor polypeptides to the cytoskeleton by a complex interaction with microtubules and microfilaments.

The regulation of synaptogenesis during normal development and following activity blockade

The mature neuromuscular junction is characterized by the tight spatial colocalization of synaptic vesicles and acetylcholine receptor (AChR) clusters. Although a large body of work exists on the interactions between motoneurons and myotubes leading to synaptogenesis in tissue culture, how the neuromuscular junction acquires its highly specialized structure in vivo is not well understood, particularly during the earliest period of synaptogenesis. In this study, the development of the neuromuscular synapse in chick hindlimb muscles was examined and quantified by simultaneously labeling the pre- and postsynaptic elements from the time the main nerve trunks leave the lumbosacral plexus region to enter the developing limb (St 24) through the end of the motoneuron cell death period (St 36). Based on these results, synaptogenesis can be divided into several distinct stages that are intimately connected to the innervation sequence described in a previous paper (Dahm and Landmesser, 1988). Briefly, as large nerve trunks approach the developing muscles and the first AChR clusters are induced to form on nearby myotubes, none of these initial receptor clusters are in direct contact with a nerve profile. The first appearance of nerve-contacted clusters (synapses) is coincident with the growth of large, unbranched nerve trunks into the muscles. The next step is initiated by the formation of small nerve side branches that grow out from the larger intramuscular nerve trunks to bring most axons and myotubes into contact for the first time. As side branches form, synapses appear around them, and non-nerve-contacted receptor clusters disappear from around the main intramuscular nerve trunks. The next step in synaptogenesis is the restriction of synaptic vesicle antigen to sites of synaptic contact. These early stages of synaptogenesis are also characterized by the growth of the presynaptic terminal to match the length of the postsynaptic receptor cluster. This study showed that AChR cluster formation during early in vivo neuromuscular development does not require close anatomical nerve contact, but that the presence of the nerves is necessary for AChR clusters to form. This suggests that the nerves normally induce AChR clustering via the release of a diffusible substance, a suggestion substantiated by the observation that AChR clusters do not form on aneural myotubes in vivo. In order to assess the role of synapse formation in the regulation of motoneuron number, synaptogenesis was quantitatively examined after chronic neuromuscular blockade, which prevents motoneuron cell death.(ABSTRACT TRUNCATED AT 400 WORDS)